Shock sensor circuitry may be used in electronic devices to detect the application of a shock or force to an electronic device. For example, the hard disk drives of portable electronic devices, such as laptop and notebook computers, may include shock sensor circuitry to detect the application of a force such as that caused by dropping or bumping the computer. If a great enough force is applied, usually greater than one G, a hard disk drive may incorrectly write data during a write operation or incorrectly read data during a read operation. This results in data errors and sometimes system failure.
The shock sensor circuitry generally senses or detects the force and generates an output signal in response. As a result of generating the output signal, an action may be taken to prevent or minimize problems caused by the force. For example, the shock sensor circuitry of a hard disk drive may generate a signal to temporarily suspend a write operation or read operation to eliminate or minimize any adverse effects that may be caused by the force.
The shock sensor circuitry receives and processes an input signal generated by a shock sensor. The shock sensor is generally constructed from a piezoelectric material and has a high impedance. The shock sensor generates an input signal at a very low voltage that corresponds to the shock or force detected. The input signal must be amplified before it is processed by the shock sensor circuitry.
Problems arise when amplifying this low voltage input signal provided from the high impedance shock sensor. Specifically, direct current leakage currents are often present in addition to the input signal. These leakage currents are amplified, along with the input signal, by an input amplifier or input stage of the shock sensor circuitry resulting in an offset voltage that creates errors within the shock sensor circuitry. These errors may result in the generation of output signals indicating that a force was detected when none was present or the generation of an output signal indicating that no force was detected when one was present.
The leakage currents may be generated by a variety of sources and are present at some level in virtually all semiconductor junctions. Oftentimes, electrostatic discharge circuitry or structures are provided at the pins of an integrated circuit. The electrostatic discharge circuitry provides enhanced circuitry protection from electrostatic discharge which can destroy an integrated circuit. Integrated circuits using MOSFET technology are particularly susceptible to electrostatic discharge damage. As a consequence of providing the electrostatic discharge circuitry, a current path is provided resulting in increased leakage currents and decreased shock sensor circuitry performance. For example, a standard operational amplifier with an electrostatic discharge circuitry at its pins may draw a direct current leakage current of ten picoamps at room temperature and from one to ten nanoamps at 125.degree. C. Leakage currents increase with an increase in temperature. Reliability suffers greatly if the electrostatic discharge circuitry must be removed to help reduce the leakage currents.
Other sources of leakage currents include current leakage from pin-to-pin across the plastic package of an integrated circuit and from the contamination present on most printed circuit boards. Printed circuit board contamination may include dust, particles, and any foreign or undesirable material on the printed circuit board that may provide a path for leakage currents. Contamination may result in leakage currents in the picoamp to nanoamp range, depending on the level of contamination. The contamination may only be eliminated or reduced by subjecting the printed circuit boards to additional cleaning processes that are both expensive and time consuming and significantly add to overall costs. Even if the contamination is initially removed, the printed circuit boards may later become contaminated resulting in increased leakage currents and decreased shock sensor circuitry performance.